This invention relates in general to shifting mechanisms for controlling the movement of a vehicle and, in particular, to a unilever system for controlling the speed and directional movement of a vehicle such as an earthmoving machine.
More specifically, but without restriction to the particular use which is shown and described, this invention relates to a unilever control for a hydrostatic transmission of an earthmoving machine for controlling speed of movement in a forward or reverse direction.
Earthmoving machines frequently utilize a hydrostatic transmission which is controlled by the machine operator through a control mechanism including a hydraulic pilot device which operates control valves for various pumps and fluid motors used in the hydraulic transmission circuit. Generally such transmission control mechanisms include a shift lever movable through a predetermined path of movement referred to as a Z-shifting pattern to shift between ranges in one direction of movement, e.g. forward, and ranges of movement in an opposed direction, e.g. reverse. Certain transmission control constructions, such as disclosed in U.S. Pat. No. 4,228,879, utilize a guide plate containing a shift pattern cutout in which the transmission lever is movable in a predetermined pattern. Such guide plates include stop cutouts requiring predetermined positioning of the shift lever in a particular manner when shifting the transmission. These cutouts constrain the shift lever for movement in one direction into a first plurality of positions in which it controls the forward travel of the vehicle, and in an opposite direction into a second plurality of positions in which it controls the reverse travel of the vehicle. Between the forward travel and reverse travel positions, the transmission shift lever is placed in an idle or neutral position in which the transmission is disengaged and, thereby, the vehicle is not driven.
In order to precisely identify the idle or neutral position, the transmission shift lever is required to be moved laterally to the direction of its speed controlling operation, in order to pass between the first and the second plurality of operating positions. In such constructions, this neutral or idle position is identified by a transversely extending slot against which the transmission lever will be engaged to halt the lever's movement in its speed controlling directional operation. Therefore, the machine operator must move the transmission shift lever to follow a path of movement which is defined by the profile of the shift pattern cutout, and which comprises a first linear portion along which the shift lever is moved to engage the first plurality of positions, and a second parallel linear portion, offset from the first, which extends in an opposed direction through which the transmission lever is moved to engage the second plurality of positions. The offset portions between the first and second parallel portions is joined by the transverse portion which defines the neutral or idle station.
In transmission control systems utilizing a shift pattern cutout, in order to shift between speed ranges in a forward direction and speed ranges in a reverse direction, there is no way in which the shifting pattern may be modified. Therefore, if it beoomes necessary to immediately reverse the direction of movement of the vehicle in an emergency, the machine operator must retrace the initial path of movement and pass through the idle or neutral position before being able to reverse the direction of the machine. This retracing operation is further hindered by the presence of the limit stops formed by the slot edges which require the machine operator to follow the Z-shape shift pattern cutout with three separate movements of the lever in different directions. The present invention provides a mechanism whereby in normal oepration a Z-shift pattern is maintained, but can be overridden by the vehicle operator in an emergency situation to shift directly between forward and reverse.